A variety of coating materials exist for optical application where it is desired to maximize absorption while minimizing reflectance of light or electromagnetic radiation. Most of these previous coating materials have certain limitations and faults.
One previous absorption material has been thick black paint, which has a tendency to crack or separate from a metal substance under thermal cycling conditions. Since many infrared instruments are subject to frequent thermal cycles, paint separation can be a significant problem. Paints, or coatings in general which have organic binders, are sometimes unsatisfactory because they are degraded by temperatures above 400.degree. C. For increased absorption at longer wavelengths, thicker layers of paint are needed. An increase in the paint thickness aggravates the cracking or separation problem. Additionally, paints outgas losing up to 10 percent by weight after vacuum exposure. Some of the chemicals which outgas can be highly corrosive to microelectronic components. Furthermore, in a cryogenically cooled optical system, outgassed chemicals may condense onto cooled optical surfaces. Moreover, most infrared absorbing paints contain carbon black, and recent experiments have shown that carbon black reacts with the five electron volt atomic oxygen prevalent at low earth orbits.
Anodized coatings have also been utilized in the context of the present invention, such as those of Wade etal, U.S. Pat. No. 4,111,762 and Porepea et al, U.S. Pat. No. 4,589,972, both of which are incorporated herein by reference.
Wade et al teaches a basic anodization method for preparation of light-absorbant materials wherein an anodizable work piece is cleaned, vapor honed, rinsed, masked, etched, anodized, rinsed, surface dyed, rinsed, hot water sealed, and (optionally) vacuum dryed. The surface prepared by this method comprises an oxide layer, e.g., Al.sub.2 O.sub.3, having an outermost dyed region of spire-like surface features.
The anodized optically black surface produced in accordance with U.S. Pat. No. 4,111,762 eliminates the concerns of outgassing and adhesion noted above. Its effectiveness on surfaces for infrared instruments depends largely upon its ability to scatter or diffuse incident radiation. Recently, there has become a need for improved infrared absorption for infrared instruments and detectors, particularly for wavelengths from about 35 microns to about 1,000 microns. If the surfaces on an infrared instrument specularly reflect infrared radiation, the resultant off-axis stray light that hits the detector can reduce the signal-to-noise ratio. Absorption in the far infrared region has become even more important now that certain detectors and instruments have improved sensitivity into the far infrared region up to 1,000 microns.
Pompea et al modify the method of Wade et al in two ways: first by adding a major surface modification step such as photoetching, prior to anodization to produce relatively large surface craters, depressions, and indentations; and second, by using steam sealing rather than hot water sealing. The resultant surface of Porepea et al has improved capability for absorbing and reducing specular reflectance of electromagnetic radiation in the wavelength range of from about 1 micron to about 500 microns or higher. The method, however, is subject to certain restrictions and limitations such as specular reflectance peak values encountered at wavelengths below 6 microns and above 1 micron, and fragility. Also, this procedure is not applicable as a coating process to be used for substrates other than the parent metal of the component.
In addition, various other electrodeposition methods are known, such as taught by Smith, U.S. Pat. No. 2,559,263, incorporated herein by reference, for providing on the surface of silver, copper, or brass plates a highly brilliant polish. This, according to Smith, is done by means of electropolishing the surface anodically in an electrolyte whereby a composite voltage made up of a direct current potential and an alternating current potential is intermittently applied to the electrodes.
In U.S. Pat. No. 3,929,563, of Sugiyama et al, incorporated herein by reference, a method is disclosed for providing a colored oxide film on an aluminum material by means of placing the aluminum in an electrolytic bath containing a metallic salt, and applying a voltage of either positive or negative pulse waveform for a period of time to produce an oxide surface layer.
Another pulse plating method is taught in U.S. Pat. No. 4,147,595, of Shigeta et al, incorporated herein by reference. Shigeta et al disclose a method for producing colored oxide films on aluminium materials by dipping the aluminum material having an anodic oxide film thereupon into an electrolytic bath containing boric acid and water soluble nickel salt, and subsequently applying alternating voltage pulses with the negative pulse voltage and duration being longer than the positive pulse voltage and duration. In this manner, Shigeta et al alternately draw coloring metal ions into the pore structures during the negative voltage cycle, and educe the coloring metal ions during the lower voltage and duration positive pulse.
In U.S. Pat. No. 4,361,630, of Johnson, incorporated herein by reference, a method is taught for producing an ultra-black surface coating with an extremely high light absorption capability on a variety of substrates, this surface coating being produced by preferential chemical attack on an electroless nickel-phosphorous alloy deposited on the substrate in a bath consisting of aqueous nitric acid solution. This resulting blackness of the surface coating is associated with a multitude of microscopic conical pores etched perpendicularly into the surface. This surface however has less than the desired absorptance of light at low angles of incidence.